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Alternative power system design

One of the first questions people ask is: "What can I run on alternative power?"

The short answer is: Anything you like!

However, let's assume the size of the system powering the home is to be 5kWhr (5000 watt hours) or under, using solar (cost effective for most people). A generator can be used for backup in times of sustained bad weather or at times when power demand is higher than usual.

It is not usually very practical to use electricity generated by renewable sources to:

  • heat water
  • cook with electrical ranges (a microwave may be ok)
  • space heat or underfloor heat
  • use heat lamps
  • basically anything that uses electricity to heat for all but a short time

Of course you can do these things off grid, but:

To do them with electricity, you will most likely have to run a reasonably sized diesel genset for the duration of time the appliance is in use.


We are currently experimenting with a vegetable oil powered machine so we can drive a few things that would otherwise tax our solar based systems batteries and run on what I consider to be an environmentally friendly & cheap fuel.

With our system at home we have a small benchtop electrical oven ( which we have to run the generator to use ) for the occasional bit of baking when the woodstove is not in use. This is usually in summer. A gas oven is another option.Solar hot water heaters are the go for your hot water requirements backed up by a wet back on the fire, gas calafont, or maybe you can tap into the waste heat from a generator.The exception is if you are extremely fortunate and have a Hydro based system producing oodles of power.

If you're lucky enough to have such a Hydo based system the output from the turbine may be such that the above does not apply. I have seen a hydro system powering a house that produced more electricity than the lucky owner knew what to do with. I was most envious! I would almost guarantee that family will never be looking at any alt. power sites. There may be wind powered sites that are almost as fortunate, but the 24/7 output from a hydro turbine is hard act to beat.

Other forms of alternative renewable i.e. solar or wind power are most suited for:

  • Energy efficient lighting
  • Energy efficient Refrigeration Call for a quote on our energy efficient models
  • Washing machines
  • Vacuum cleaners (we usually run our gen set when using ours)
  • Entertainment
  • Office equipment like computers, fax etc (not the coffee maker if it stays on all day) expresso machine (uses about 100watts to make a cup of coffee, seems I have a 500watt/hr a day habit :-)
  • Water pumping
  • Power Tools

The most common way for a designer to size a system is to ask the clients to fill out a sheet which details the appliances they are likely to use, the power consumption of each device, and the length of time each device is likely to be in use for per day.

The reason for this is really quite simple. It is simply an energy audit, or accounting for your proposed energy use. It's a bit the same as doing a financial budget and some say about as difficult! but don't worry here's some pointers

  The power consumption of a device is equal to the length of time it is on, times the power it draws, e.g. a television that is rated at 100w is left on for 4 hours it will consume 400 watt/Hours.

4hours x 100 = 400watt/hrs

The same T.V on for 2 hours : 2hours x 100watts = 200 watt/hrs


Great! we have the telly sorted now we have to go through the same exercise for everything else in the house that uses electricity!

Once the designer has gone through the energy budget they can design the system using these figures. It does work if every appliance has been noted and the times suggested by the customer are reasonably accurate.


Not many people grid based people really think or are even aware of what they are using and how long for until they actually find themselves OFF THE GRID! (I know I didn't!). For example, do you have a water pump? If so, how long does it run for? How long do you watch the TV for on an average night, and what's an "average night"? Its like asking how long a piece of string is. If you get into the mindset after a little practice you will be much more aware of your energy consumption and after a while it's easy! second nature you could say. Grab yourself a notebook and pencil and your away.

But for an example here and now I'll use myself

If you're like me and like to watch a few movies & Doco's on the box, prefer to have an electric fridge and say 300 liter chest freezer, intend on using a computer, vacuum cleaner, washing machine and kitchen appliances, I think you need to be looking at a system that will give you around 3-5 kW/hrs a day (3000-5000 watt/hrs), or be prepared to use a generator when necessary on a smaller system

  Please note:

When I'm talking electric refrigeration, I mean efficient, well insulated units that use less than half the energy of the normal brands we are all used to.

They cost a bit more, but the savings they make in consumption of your alternatve power pays off big time, as the generating and storage systems i.e. Pv Arrays & Batteries can be smaller.

2012 many of the fridges now on the market are very efficient compared to those of a few years ago. Its easy to find the good ones now that they have to have an energy efficiency rating displayed on them. Generally speaking if you can find one you like that uses between say 200-300kWHrs a year, its going to e suitable for a modest size offgrid system. Of course the lsee it uses the better off you are :-)

I have looked at the gas option but they are difficult to find apart from the real cheapies, which all seem to be pretty hungry on gas which in the last few years has got way more expensive.


If you find some other means of refrigeration you may be able to halve this, and if you don't have T.V. square eyes like me, you can probably take off another 3-400 watthrs.If you are going to buy a new television LCD types tend to use less power

2012 I saw a lovely big plasma at a friends the other day which uses not much more than my 32 inch LCD, which is only a few years old, but is near TWICE the size. Technology has really moved ahead here, so again look for one you like with a good energy rating

As you can see, lifestyle determines how much power generation is needed as does the number of people in the family and their individual power requirements. I am expecting our consumption to increase as my young son matures and wants to use more electronic devices. Currently, I am losing quite a few watts to "Bob the Builder" & his mates (2007). These days its cartoon network and his computer

2012 re the preceeding paragraph, Yup I was right starwars and runescape now :-)

To read more on calulating power use please click here


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